Does Blender have the equivalent of Photoshop Smart Guides?
"Snap" is what you're looking for, but the way to use it isn't really intuitive if you come from Photoshop. Please let me detail the procedure step by step.

Starting with the default config :


Set the "Snap element" to Vertex
Start to extrude with E. If needed, specify an axis (X in your case)
Keep Ctrl pressed, and move your mouse cursor to either A or B. When the face snaps, press left mouse button to validate.